1. Field of the Invention
The present invention relates to an oscillation circuit having smaller oscillation frequency variation caused by power supply voltages or ambient temperatures.
This is a counterpart of Japanese patent application Serial Number 273643/2006, filed on Oct. 5, 2006, the subject matter of which is incorporated herein by reference.
2. Description of the Related Art    Patent document 1: Japanese Patent Kokai Number 2003-4547    Patent document 2: Japanese Patent Kohyo Number 2005-533443
In the above document 1, there is a description of the temperature detecting circuit for outputting a temperature signal by comparing frequencies between the ring oscillator changing the oscillation frequency corresponding to the ambient temperature and the crystal oscillator.
Furthermore, in the above document 2, there is a description of the constant current circuit for supplying a constant current independently from the power supply voltage or the ambient temperature and the current-control type ring oscillator configured by a ring oscillator driven by the above constant current circuit in order to output a constant oscillation frequency.
It is publicly known that a frequency of ring oscillator dependents on an ambient temperature largely as described in the patent document 1, and then a oscillation frequency is stabilized by making the current supplied to the ring oscillator constant, as in the patent document 2. However, since a oscillation frequency of ring oscillator depends on not only the power supply voltage but the ambient temperature also, it is impossible to stabilize the frequency completely only by making the current constant.